Japanese gates are normally defaulted to open and close if your ticket isn't valid or you don't insert one. I once did push through one - the way we were using the system we were buying the cheapest ticket and using the "adjust" machine (basically an excess) to pay up to the correct fare before exiting - this is a bit simpler than trying to understand the fares and is generally how tourists were traditionally advised to do it.
However what we had done was gone in and ridden round a loop, exiting at the same station, something it couldn't cope with and it wouldn't take the excess fare. With no staff visible there was no option but to push through as the machine wouldn't take any extra fare - I did think someone would come out of a door somewhere and stop me, but nobody was there and nothing happened aside from a loud alarm that we presumed was intended to embarrass any evader into not doing it, a bit like how a burglar alarm works.
We had every intention of paying (and thought it would just excess up to the maximum from that station) but it just wasn't a scenario the system had been designed for. (Oyster by contrast does charge a fare and let you out if you do a same station exit e.g. if you go all the way round the Circle Line, I seem to recall it's different depending on how long you leave it).